The present invention relates to a lens driving device for a camera and, more particularly, to a device for correcting a position control error caused by a backlash of a driving mechanism between a lens and a driving means provided for driving the lens.
A plurality kind of backlash correction is disclosed in Japanese Patent Laid-Open Application No. 106009/1984 and Japanese Patent Laid-Open Application No. 115217/1977.
In the former application, a device is proposed in which a direction of a torque generated by a driving motor is detected and a start point of a backlash correction is determined in response to reversing timing of the torque. By this correction, the backlash is corrected in deceleration (braking) after acceleration. Although this correction is suitable for a device such as a machine tool, having a driven member with high inertia, it is not suitable for a device such as a camera having a driven member with low inertia such as a lens. The reason is that the direction of the backlash is not changed in deceleration of the torque of the driving motor, because the inertia of the lens is about 1/10-1/100 lower than that of the driving motor and viscous resistance material such as grease is used in a driving mechanism. Therefore, it is difficult to apply the backlash correction disclosed in the former application to cameras.
In the latter application, a device is proposed in which an optical system is stopped at one of a plurality of predetermined positions only when the system is moving in a specific direction. It can not be applied to a device in which the stop positions can be optionally set, because the stop positions are predetermined in the device of the latter application. Further, the optical system of the latter application moves or is driven only in one direction, so that controlling the device is strongly restricted.
Meanwhile, a camera needs lens position control with an output of a detector provided for detecting a lens position through an automaticfocus (AF) coupler provided between the lens and the driving means. However, a large backlash exists in the AF coupler because the coupler is mounted in a driving mechanism between the detector and the lens. Therefore, accurate control of the lens becomes insufficient so that a picture may be taken in out-of-focus condition.